The present invention relates to polymeric materials having enhanced ion and water transport property, medical devices comprising such materials, and methods of making such materials and devices. In particular, the present invention relates to ophthalmic devices having enhanced ion and water transport property.
Advances in chemistry of materials for medical devices have increased their compatibility with a body environment and the comfort for their extended use therein. The extended use of contact lenses requires that materials for these lenses allow sufficient rates of transport of oxygen to the cornea to preserve its health because the cornea does not have blood vessels for the supply of oxygen and must receive this gas by its diffusion through the epithelial layer on the outer surface of the cornea. On the other hand, the cornea continuously regulates its thickness by actively pumping ions in or out of the cornea to counterbalance a continuous leak of fluid into the corneal stroma. A net flux of sodium ions from the stroma to the anterior chamber has been measured in animal models (see, e.g., S. Hodson et al., Exp. Eye Res., Vol. 11, 249-253 (1977); J. A. Bonanno, Prog. in Retinal and Eye Res., Vol. 22, 69-94 (2003)). Thus, contact lenses for extended use also should allow sufficient rates of ion transport therethrough. Moreover, in view of the need rapidly to regulate the cornea thickness, the desirable materials should have an ion transport rate as high as possible. Although materials have been developed that show high oxygen permeability, those having remarkable ion permeability have not been noticed.
While there exist rigid gas permeable (“RGP”) contact lenses, which have high oxygen permeability and which move on the eye, RGP lenses are typically quite uncomfortable for the wearer. Thus, soft contact lenses are preferred by many wearers because of comfort. (Soft materials are those exhibiting low modulus of elasticity, such as less than about 150 g/mm2.) Moreover, a contact lens which may be continuously worn for a period of a day or more (including wear during periods of sleeping) requires comfort levels that exclude RGP lenses as popular extended-wear candidates. Among the soft contact lens materials having high oxygen permeability have been polymers containing siloxane groups. For example, see U.S. Pat. Nos. 3,228,741; 3,341,490; 3,996,187; and 3,996,189. However, polysiloxanes are typically highly hydrophobic and lipophilic. The properties (e.g., lipophilicity, glass transition temperature, mechanical properties) of known polysiloxanes have resulted in contact lenses which adhere to the eye, inhibiting the necessary lens movement. In addition, polysiloxane lipophilicity promotes adhesion to the lens of lipids and proteins in the tear fluid, causing a haze, which interferes with vision through the lens.
Therefore, there have been efforts to develop hydrophilic polymers, which have both high hydrophilicity and high oxygen permeability. Such polymers typically combine a hydrophilic monomer (such as 2-hydroxyethyl methacrylate (“HEMA”), N-vinyl-2-pyrrolidone (“NVP”), N,N-dimethyl acrylamide (“DMA”), methacrylic acid “MAA”), or acrylic acid) and units of siloxane-containing monomers. For example, see U.S. Pat. Nos. 3,808,178; 4,136,250; and 5,070,169. These polymers typically are random copolymers. Other works have been directed to develop block copolymers, such as those consisting of polysiloxane and polyoxyalkylene blocks. See, for example, EP 267158, EP 330615, EP 330616, and EP 330617.
Although there have been attempts to develop materials for contact lenses that have both high oxygen permeability and high ion transport rate, such materials have not been apparent. For example, U.S. Pat. Nos. 5,807,944 and 5,849,811 disclose polymers comprising blocks or segments of polymers having high oxygen permeability and blocks or segments of polymers that are said to have high ion permeability. The oxygen-permeable blocks comprise a siloxane-containing macromonomer, such as polydimethylsiloxane that may include hydrophilic groups. The ion-permeable blocks comprise units of a typical hydrophilic monomer that has been used to synthesize hydrophilic polymers, including the monomers disclosed above or cyclic ethers having only one oxygen atom in the ring. Although a range of ion diffusion rates through these materials was achieved, these rates may still be inadequate for the cornea health, and higher rates are still desirable.
Therefore, there is a continued need to provide other materials for medical devices in general, and contact lenses in particular, that have both improved oxygen permeability and ion transport rate. It is also very desirable to provide materials for such devices that have improved oxygen permeability and ion and water transport rates.